Bone loss is a major consequence of spaceflight as well as aging and disuse. These conditions are marked by a decrease in the osteoblastogenesis of mesenchymal stem cells and an increase in adipogenesis. Cbfa1 is an essential transcription factor in osteoblast differentiation and bone formation. In addition, PPAR-gamma was shown to be crucial for adipocyte differentiation. Given this strong relationship between osteogenesis and adipogenesis, we hypothesize that modeled microgravity disrupts integrin-mediated signaling leading to alterations in the expression and/or activation of both Cbfa1 and PPAR( in human mesenchymal stem cells (hMSC), which results in suppressed osteoblast differentiation and increase adipogenesis. To test this hypothesis, hMSC will be grown in a Rotary Cell Culture System with a high aspect ratio vessel. The aims of this study are: (1) Characterize the effects of modeled microgravity on human mesenchymal stem cell differentiation. Modulation of osteoblastogenesis and adipogenesis will be examined by alkaline phosphatase, von Kossa, oil red O and trichrome staining. RNA analyses will be performed to demonstrate the modulation of osteoblastic and adipogenic gene expression. (2) Characterize the molecular mechanisms responsible for the effects of modeled microgravity on human mesenchymal stem cells. Western blot and gel shift analysis and transactivation studies will be utilized to elucidate the mechanism. (3) Identifying agents capable of reversing the modeled microgravity inhibition of osteoblastogenesis. Using the same techniques employed in Aims 1 and 2, we will identify agent/s capable of blocking microgravity-induced bone loss. Findings from these studies will provide insight into mechanisms involved in the effect of weightlessness on mesenchymal cell function, thus addressing the requirements of this PA (PA-00-088). A viable basis will thereby be provided for the development of drugs that can alter Cbfa1 and/or PPARgamma and mediate increased bone formation in astronauts, elderly and patients with disuse osteoporosis.